On the rational design of mesoporous silica humidity sensors

Mesoporous silica is commonly used as matrix for humidity sensors, which operate on the principle of relative humidity-dependent water uptake and read-out by resistive or capacitive means. Although numerous studies have been dedicated to improving the sensing performance, in particular with conductive additives, the effect of pore structure on sensing behaviour has not been systematically investigated so far. Herein, we showcase the effects of pore size and porosity on resistive sensing behaviour in the 0.5-85% relative humidity (RH) range. We employed evaporation-induced self-assembly (EISA) in combination with sol-gel chemistry to fabricate well-defined mesoporous silica thin films with high degree of structural control. Material architectures with pore sizes of 3 to 15 nm and porosities of 40 to 70% were rationally designed by using structure directing agents (SDAs) with increasing molecular weight and tuning the silica to SDA ratio. We found that a combination of pore size of 15 nm and 70% porosity showcases a particularly high sensitivity (~104 times change in resistance) in the measured range, with quick response and recovery times of 3 and 9 seconds, respectively. Across the various sensors, we identified a clear correlation between the pore size and the linear RH sensing range. Sensors with larger mesopores (~15 nm) exhibited higher sensitivity and linear response in the 65 to 85% RH range than sensors with smaller pores (<8 nm). Additionally, increasing the porosity while retaining the pore size, yields better overall sensitivity across the range. Our findings may serve as guidelines for developing broad spectrum high-performance mesoporous sensors and for sensors specifically engineered for optimal operation in specific RH ranges.